1. Field of the Invention
The present invention relates to a method of calibrating a semiconductor line width, by which an irregular line width bias generated from exposure and etch in semiconductor fabrication is accurately corrected.
2. Discussion of the Related Art
Generally, a photomask pattern directly affects the precision of a real pattern formed on a semiconductor substrate. If an optical proximity effect of the photomask pattern fails to be correctly taken into consideration, a pattern line width is distorted from the intended exposure of the photolithography. Hence, line width linearity is shortened, degrading the semiconductor device characteristics.
In semiconductor photolithography, a precise design of a photomask enables a quantity of light transmitted via the photomask to be appropriately adjusted. For the precise design of the photomask, OPC, phase shift masking, and the like has been proposed, as well as various methods for minimizing the light distortion attributed to a shape of the pattern drawn on a mask.
Lately, a chemically amplified resist having excellent sensitivity to far infrared ray wavelength (248-194 nm) has been developed to substantially enhance resolution. The resolution enhancement is attributed to the technique of forming a supplementary (dummy) pattern separated from a primary pattern to control an optical proximity effect.
The most important fact is to lower a bias between a line width CD (critical dimension) and a design CD according to an exposure machine. The bias between the CDs tends to occur optically. A micro loading effect between patterns takes place via an etch process. The CD bias problem is attributed to byproducts such as polymers and the like.
The former line width bias is called development image (DI) CD bias and the latter line width bias is called final image (FI) CD bias.
The DI CD bias is predictable by a program copy method which generates a model capable of predicting a bias correction method. On the other hand, the FI CD bias is difficult to predict by the program copy method. Specifically, since the FI CD bias is represented by expressions of chemical composition and reaction, it is difficult to produce reproducible and correct results. Moreover, the FI CD bias tends not to follow the tendency of the DI CD, instead showing a new tendency. Hence, it is very difficult to generate a FI CD pattern forming model.
FIG. 1 shows a calibration mask having line widths adjusted by step in a semiconductor fabrication method according to a related art. A general illumination system having 248 nm DUV scanner, 0.65 N.A., and 0.55 sigma as exposure conditions and in which ‘TOK-P028 750 nm’ is used as photoresist for spin coating.
Referring to FIG. 1, a mask consists of a plurality patterns of isolated line parts 1A, 2A, and 3A, an isolated face part 30A, and dense line parts 1B-1C and 2B-2C adjusted by pitch. The pitch adds a line and space.
FIG. 2 is an exemplary graph of plotted line width pitches in FIG. 1 based on DI CD and FI CD exposed via the mask in FIG. 1.
A DI CD error 5 is a value plotted per pitch for a value resulting from subtracting a design CD from a DI CD exposed and developed via the corresponding mask.
A FI CD error 6 is a value plotted per pitch for a value resulting from subtracting a design CD from a line width CD after the completion of the etching.
Moreover, a design CD error 4 corresponds to a reference line. A line width bias A-B between the corresponding DI and FI CDs has no fixed difference according to a pitch.
In other words, DI SA and FI 6A considerably differ from each other in tendency at 1,000 nm pitch. As the pitch increases, FI CD 6B keeps increasing but DI CD 6A becomes saturated.
Therefore, it is important to express the tendency of FI CD by numerical formula.